The invention concerns a method of preparing 3-bromophthalide, also called 3-bromo-1(3H)-isobenzofuranone, by the reaction of orthotoluic acid with bromine at elevated temperatures.
3-Bromophthalide is a known compound (Beilstein 17, p. 312, I p. 162, II p. 334) which can be prepared by the bromination of phthalide either with elemental bromine or with N-bromosuccinimide (Organic Syntheses 23 (1943) 74, 42 (1962) 26). By direct bromination, yields of 82 to 83% of the theory are obtained, but by bromination with N-succinimide yields of only 75 to 81% of the theory of 3-bromophthalide are obtained, and the product must then be recrystallized for purification.
Bromination with phthalic dialdehyde in carbon disulfide to 3-bromophthalide with only a 60% yield has been described by Simonis (Berichte 45 (1912) 1584).
Disadvantages of these methods are the insufficient purity, which can be improved by recrystallization with additional losses of yield, and the use of expensive starting substances which make these methods uneconomical.